1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting system information in a mobile communication system, and more particularly, to a method and apparatus in which a terminal incapable of multi-antenna reception may access a cell supporting multi-antenna transmission and receive system information from the cell in an Orthogonal Frequency Division Multiplexing (OFDM) system.
2. Description of the Related Art
Mobile communication systems have been developed to provide voice services while ensuring the mobility of users. The mobile communication systems have gradually expanded their coverage to data services from the voice services, now making it possible to provide high-speed data services. However, in the currently available mobile communication systems, the systems suffer from lack of resources, and because users require services having higher speeds, more advanced mobile communication systems are required.
Long Term Evolution-Advanced (LTE-A) is one of the next-generation mobile communication systems, which have been developed in response to these demands, and its standardization is underway in the 3rd Generation Partnership Project (3GPP). LTE-A is a technology for implementing high-speed packet-based communication having a data rate of a maximum of 1 Gbps. To this end, several methods have been discussed, such as a method of multiplexing the network structure so that multiple base stations may cover a specific area in an overlapping manner, and a method of increasing the number of frequency bands supported by one base station.
Recently, it has become possible to easily obtain and deliver the necessary information anytime and anyplace by connecting all things around us over the network, and Machine-to-Machine/Inter of Things (M2M/IoT) that enables provision and usage of various services based thereon is the key issue for the next-generation communications market. M2M has started with the sensor and Radio Frequency Identification (RFID) networks that mainly cover local areas, but due to the growing diversity of its applications and features, it may be used for various wired/wireless networks. The interest in M2M based on mobile communication networks has increased in consideration of the mobility of objects, wide service areas, ease of operation and maintenance of the networks, security for high-reliability data transmission, and guarantee of service qualities. For these data, very small packets need to be transmitted, and their transmission cycle is very long. About 30,000 of such terminals (or M2M terminals) can exist in one cell, so the existing base station that handles hundreds of general terminals (or mobile terminals) has significantly increased in terms of the number of terminals it should handle.
Reflecting this, the 3rd Generation Partnership Project (3GPP), has proceeded with the full-scale standardization work with a name of Machine Type Communications (MTC) since 2008, starting with a feasibility study for M2M in 2005. These MTC terminals cannot receive multi-antenna transmission since they are produced at a low cost. In other words, if having multiple antennas, a base station transmits a control channel using multiple antennas, but the MTC terminal may not access the base station because it cannot receive this multi-antenna transmission. If the base station performs single-antenna transmission to all of its terminals even though it has multiple antennas, coverage of its cell is reduced, so the existing terminals (i.e., mobile terminals) may suffer from degradation of the reception quality.
An OFDM transmission scheme, a multi-carrier scheme of transmitting data using multiple carriers, is a kind of Multi Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbol streams, and modulates each of the parallel symbol streams with multiple carriers (i.e., multiple sub-carrier channels) based on the orthogonal relationship between them, before transmission.
A system employing MCM was first applied to military high-frequency radio receivers in the late 1950s, and the OFDM scheme, in which multiple orthogonal subcarriers overlap, has begun to be developed since the 1970s. However, the system employing MCM had a limitation on its application to the actual systems because of the difficulty in implementing orthogonal modulation between multiple carriers. Nevertheless, development of the OFDM technology has proceeded rapidly since Weinstein et al. announced in 1971 that the OFDM-based modulation/demodulation can be efficiently processed using Discrete Fourier Transform (DFT). In addition, with the introduction of the new approach of using a guard interval and inserting Cyclic Prefix (CP) symbols in the guard interval, the negative impacts of the multipath and delay spread on the systems have been further reduced.
Thanks to the development of these technologies, OFDM technology is now widely applied to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). In other words, the OFDM scheme, which was not widely used due to its hardware complexity, may now be implemented with the development of various digital signal processing technologies including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT).
Although similar to the conventional Frequency Division Multiplexing (FDM) scheme, the OFDM scheme may obtain its optimal transmission efficiency during high-speed data transmission by maintaining the orthogonality among multiple tones. In addition, the OFDM scheme may obtain the optimal transmission efficiency during high-speed data transmission because it has high frequency efficiency and is robust against multipath fading.
As another advantage of the OFDM scheme, the OFDM scheme has high frequency efficiency and is robust against frequency selective fading and multipath fading since it uses the frequency spectra in an overlapping manner, and with the use of a guard interval, the OFDM scheme can reduce Inter Symbol Interference (ISI), ensure simple design of the hardware structure of an equalizer, and is robust against impulse noises. Thus, the OFDM scheme is widely used in the communication system structure.
In wireless communications, high-speed high-quality data services may be interrupted generally due to channel environments. In wireless communications, the channel environments may vary frequently due to the change in power of received signals, caused not only by Additive White Gaussian Noise (AWGN) but also by fading; the Doppler effects based on shadowing, movement of terminals, and frequent changes in speed of terminals; and interference by other users and multipath signals. Therefore, these factors worsening the channel environments need to be effectively reduced in order to support the high-speed high-quality data services in wireless communications.
In the OFDM scheme, a modulation signal is located in two-dimensional resources consisting of time and frequency resources. Resources in the time domain are distinguishable by different OFDM symbols, which are orthogonal with each other. Resources in the frequency domain are distinguishable by different tones, which are also orthogonal with each other. In other words, in the OFDM scheme, one minimum unit resource may be indicated by designating a specific OFDM symbol in the time domain and a specific tone in the frequency domain, and the unit resource is called a Resource Element (RE). Different REs are orthogonal with each other even after experiencing a frequency selective channel, so signals transmitted with different REs may be received at a receiver without mutual interference.
A physical channel is a channel of a physical layer, which carries modulation symbols obtained by modulating one or more coded bit streams. An Orthogonal Frequency Division Multiple Access (OFDMA) system configures and transmits multiple physical channels differently depending on the usage and receiver of transmission information streams. For each physical channel, a transmitter and a receiver should agree in advance to determine to which RE they will match to the physical channel, and this rule is called ‘mapping’.
Therefore, there is a need for a method in which a base station with multiple antennas may support both of terminals capable of multi-antenna reception and terminals incapable of multi-antenna reception (i.e., the terminals capable of single-antenna reception).